Halos, shadows and new engines

Last updated on September 16, 2023

Halos, Shadows, and New Engines: Unveiling the Mysteries of Atmospheric Optics

Atmospheric optics is a captivating field that explores the fascinating phenomena occurring in our skies. One intriguing topic within this realm is the interaction between halos, shadows, and contrails produced by aircraft. In this article, we will delve deeper into this subject and uncover the intricate connections between these atmospheric marvels.

The Dance of Halos and Shadows

The captivating images captured by Peg Zenko, Christopher Gerekos, and Ian Brock have shed light on the mesmerizing interplay between contrails, shadows, and halos. In these photographs, we witness bright contrails gracefully traversing the sky, accompanied by their enigmatic shadows and the ethereal presence of a 22° halo. But are these elements truly connected?

The answer is both yes and no. In the cases presented, the aircrafts generating the contrails are cruising at high altitudes, with their trails positioned above a thin layer of translucent cirrus clouds capable of producing halos. The dark trails we observe are actually the shadows cast by the contrails onto the cirrus layer below. Astonishingly, these contrails can be situated thousands of feet above the crystals responsible for halo formation.

Ian Brock's fortuitous position between a contrail and its shadow on lower clouds provided a unique perspective. This vantage point allowed for a deeper understanding of the apparent position of contrail shadows relative to the trails themselves. Surprisingly, due to the parallel nature of the sun's rays, perspective plays a role in making the shadow appear farther away from the sun than the actual contrail. When a contrail passes in front of or closely aligns with the sun, its shadow can race ahead as a dark spike in the sky.

The Contrail Conundrum

An intriguing question arises: are there more contrails in our skies today compared to the past? The answer lies in a combination of increased aircraft movements and advancements in aviation technology.

Undeniably, the rise in global air traffic has contributed to an uptick in contrail sightings. With more planes traversing our skies, the likelihood of encountering these atmospheric phenomena naturally increases. However, there is another, somewhat paradoxical factor at play.

Modern aircraft, equipped with fuel-efficient engines and airframes, have the ability to form contrails under a wider range of atmospheric conditions than their predecessors. This may seem counterintuitive at first glance. How can more efficient engines lead to an increase in contrail formation?

The key lies in the way modern engines utilize the combustion heat produced during flight. Unlike older aircraft, which often wasted a significant portion of this heat in exhaust gases, newer engines are designed to optimize fuel efficiency. As a result, the plumes emitted by these engines are cooler and possess a higher relative humidity. These conditions are conducive to the condensation of moisture, leading to the formation of contrails.

A Glimpse into the Science

To further illustrate the impact of propulsion efficiency on contrail formation, let us examine a scientific trial conducted by researchers in the field. In the accompanying image, we observe a comparison between a modern Airbus A340 and an older Boeing 707 flying under identical conditions.

The striking visual depicts a striped pole, representing the nose probe of the research aircraft following closely behind. In this experiment, it becomes evident that the Airbus A340 produces contrails, while the aging Boeing 707 remains devoid of these atmospheric trails. This stark contrast highlights the influence of propulsion efficiency on contrail formation.

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In conclusion, the captivating dance between halos, shadows, and contrails unveils a world of wonder within atmospheric optics. Through the lens of talented photographers and scientific investigations, we gain a deeper understanding of the intricate connections between these phenomena. As aircraft technology continues to evolve, the skies above us will undoubtedly continue to reveal new and mesmerizing displays of atmospheric beauty.

Halos & Shadows

1. Peg Zenko (Tangent Photos) - Illinois

2. Christopher Gerekos - Belgium

3. Ian Brock - Atlantic Ocean

In Peg Zenko's (1) and Chris Gerekos's (2) images, bright contrails cross the sky accompanied by their dark shadow and a 22� halo. Are they connected? Yes and no. In both cases the aircraft is at cruising altitude and its contrail is above a thin and translucent layer of halo producing cirrus cloud. The dark trail is the shadow of the contrail cast downwards onto the cirrus layer. The contrail can be thousands of feet above the halo forming crystals.

Ian Brock (3) was fortunate to be between a contrail and its shadow on lower clouds.

The apparent position of contrail shadows relative to the trail itself can be counterintuitive. The sun's rays are everywhere parallel and perspective makes the shadow appear farther away from the sun than the contrail itself. When a contrail passes in front of the sun or is very close its shadow can race ahead of it in the sky as a dark spike.

Are there more contrails than there used to be? Yes, aircraft movements have considerably increased.

But there is another reason - Paradoxically, modern aircraft with more fuel-efficient engines and airframes form contrails under a wider range of atmospheric conditions than did older airplanes.

Modern aircraft waste less of the fuel's combustion heat in the exhaust gases. The resulting cooler plumes have a higher relative humidity and their moisture condenses more readily to form contrails.

The lower image (4) is of a scientific trial (Aerosp. Sci. Technol. 4 (2000) 391-401 'Influence of propulsion efficiency on contrail formation'). On the left is a modern Airbus A340 - on the right is an old Boeing 707. The striped pole is the nose probe of the following research aircraft. The aircraft are flying under identical conditions. The Airbus produces contrails, the older 707 does not.

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